Photodiode
Photodiode is a kind of
diode which generates current proportional to the incident light energy. It is a light to voltage/current converter that finds applications in security systems, conveyors, automatic switching systems etc. The photodiode is similar to an LED in construction but its p-n junction is highly sensitive to light.
The p-n junction may be exposed or packaged with a window to enter light into the P-N junction. Under the forward biased state, current passes from the anode to cathode, while in the reverse biased state, photo current flows in the reverse direction. In most cases, the packaging of Photodiode is similar to
LED with anode and cathode leads projecting out from the case.
There are two kinds of Photodiodes – PN and PIN photodiodes. The difference is in their performance. The PIN photodiode has an intrinsic layer, so they must be reverse biased. As a result of reverse biasing, the width of the depletion region increases and the capacitance of the p-n junction decreases.
This allows the generation of more electrons and holes in the depletion region. But one disadvantage of the reverse biasing is that, it generates noise current that may reduce the S/N ratio. So reverse biasing is suitable only in applications that require higher band width. The PN photodiode is ideal for lower light applications because the operation is unbiased.
The Photodiode has a p-n junction or PIN structure. The P-N junction is made up of a light sensitive semi conductor. When the photon energy excites the p-n junction, the electrons will be mobilized and the holes are produced. This process occurs in the depletion region of the p-n junction.
The holes then move towards the anode and the electrons towards the cathode. This generates the photocurrent in the Photodiode. Silicon (190-1100 nm), Germanium (400-1700 nm), Indium Gallium Arsenide (800-2600 nm), Lead Sulphide (1000-3500 nm) etc are the semiconductors used for making different types of photodiodes.